Method of applying an electrically conductive transparent coating to a nonconductivebase



April 9, 1957 s. LOUIS 2,788,296 THOD OF APPLYING AN ELECTRICALLYCONDUCTIVE TRANSPARENT COATING TO A NON-CONDUCTIVE BASE Filed Nov. 15.1951 HIE INiENTOR. HEN ULIJ S. LULIIS FlEiENT United States PatentMETHOD OF APPLYING AN ELECTRICALLY CON- DUCTIVE TRANSPARENT COATING TO ANON- CONDUCTIVE BASE Arnold S. Louis, New York, N. Y., assignor to MyronA. Coler, Scarsdale, N. Y.

Application November 15, 1951, Serial No. 256,515

8 Claims. (Cl. 117---211) 1 This invention relates to electricalinsulators having transparent electrically conductive surfaces andmethod of making same. I

In many applications for electrical insulators such as glass andplastics it is desirable that an electrically conductive surface beprovided in order to eliminate electrostatic charges. In electricalmeasuring apparatus electrostatic charges cause disturbances tosensitive meters. Accordingly, a conductive surface is sought for themeter casing observation window. In another typical case, that ofaircraft, the electrostatic charging of the plastic cockpit canopy andinsulating plastic surfaces covering radio and radar antennas createelectrical interference with radio communication, and radar signals.

There is disclosed herein a simple method of coating a thermoplasticarticle with a transparent conductive surface involving the applicationof sufficient heat and pressure to secure a relatively firm bond betweenthe conductive material and the insulating base. The conductivematerials distinctly pointed out in the examples which follow includecarbon which is well known in the prior art as a useful coating materialhaving great hiding power but not as a transparent material.

The prior art shows various partially successful methods of applyingcarbon and other conductive films. Thus, surfaces possessing sutlicientinherent adherence such, for instance, as that of a wax object, willreadily attach to itself a relatively continuous graphite layer whichcan be electroplated. Such films are rather delicate and usuallycompletely opaque. Again, it is conventional to apply graphite indispersion in film-forming adhesives to the surface of insulatorobjects. It has not been possible to obtain such coatings which aresufficiently conductive and, at the same time, reasonably transparent.Furthermore, there are problems connected with peeling and looseadherence of the coating and with crazing of insulator plastics to whichthey are applied.

Aconventional device to obtain transparent conductive coatings is theapplication of organic films containing water-soluble electrolytes. Suchfilms are generally very unstable as to their electrical properties andcan be easily washed from the insulator base. This invention,

by contrast, is concerned with obtaining a coating which is sufiicientlythin to be transparent and yet sufficiently adherent to withstandconsiderable abrasion and washing.

i Accordingly, it is an object of this invention to provide an improvedmethod for applying an electrically conductive transparent coating to athermoplastic insulator.

It is another object of this invention to provide a method for applyingan electrically conductive transparent coating to a transparentthermoplastic material.

A still different object is to provide a glass having an electricallyconductive transparent surface.

" A' particular object of this invention is a method of obtaining auniform conductive coating on a plastic surface.

Still another object of this invention is to provide an article moldedof a transparent plastic insulator, said article having a transparentelectrically conductive coating adherent to its outside surface.

In carrying out the present invention pressure and heat are utilized tocause the conductive material to adhere to the substrate. There is alsodisclosed in greater detail hereinafter an improved method of securingsuch coatings by a transfer or offset process in which the conductivematerial is applied to a relatively inert surface and is thentransferred by the action of heat and pressure to the substrate. Theresultant film is characterized by thinness, by relatively highadherence to the substrate and by resistance to abrading action.

For a more complete understanding of our invention and for furtherobjects and advantages thereof, reference should now be made to theaccompanying description taken in conjunction with the drawing in which:

Figure 1 diagrammatically illustrates, a. system of carrying out onephase of the invention.

Figure 2 shows in elevation a view of elements of this inventionassembled in a molding press.

Figure 3 shows in elevation a continuously operating embodiment of theprocess of this invention.

With reference to Figure .1, plate 2 is provided with a surface havinglow intrinsic adhesion to carbon, mirrorfinished chrome plate beingsuitable. Glass is also useful. What is essential'is that plate 2 havean exceedingly smooth, non-adherent surface which will not deform at thetransfer temperature. A burner 4 is adapted to burn a mixture of acarbon-containing fuel such as acetylene, ethylene, natural gas or citygas with less than the amount of air or oxygen gas required for completecombustion under conditions whereby the undecomposed fuel is thermallycracked in the flame and carbon is formed. The resulting carbon black isdeposited on plate 2. Various methods may be used to obtain a uniformcoating including use of a plurality of burners and movement of theplate or burner. The method I prefer is the use of a high voltagegenerator, 6 which charges plate 2 electrostatically so that the carbonparticles are attracted to the more thinly coated portions of the platethus providing a uniform coating.

The coated plate is then placed in close contact with a sheet of plastic8 so that the coating it) is interposed between the plate 2 and plastic8. This assembly is then compressed between heated press platens 12 and14. The proper pressure and temperature will, of course, be determinedby the characteristics of the substrate plastic used. In general, theassembly should be heated to the conventional molding temperature of theplastic, if the plastic is of the thermosetting type. If it is athermoplastic it should be heated at least to its heat distortion point,preferably to its compression molding temperature. Pressure should beabout the same as during usual molding operations, 1000 to 5000 poundsper square inch usually being adequate. in the case of thermoplastics,the articles should be held under high pressure for a comparativelyshort time, preferably only long enough to bring the surface of thearticle to the temperature indicated above.

When films are formed in this fashion it has been found that if auniform deposit of carbon is put on the transfer plate a similarlyuniform film Will be transferred to the plastic surface. Surprisingly, avery thin deposit of carbon can be laid down and transferred directly tothe plastic such that the film is actually transparent but yet retainssufiicient conductivity to discharge static readily.

For each plastic and each temperature and pressure of transfer there isa characteristic amount of carbon which will be very firmly bound to theplastic. If it is attempted to transfer a larger amount of carbon black,the remainder will be more loosely bound and can, if desired, be removedby rubbing or bufiing. A smaller amount of carbon black than thiscritical amount can be directly and firmly transferred.

Curved surfaces may be similarly treated with suitably shapedcompression dies. Alternatively, sheet may be coated prior to shaping;the coating will remain conductive after a reasonable amount of physicalwork.

Example 1 A mirror-finished chromium plate on steel was lightly butcompletely covered with carbon black from a flame formed by burning amixture of acetylene gas with less oxygen than the amount required forcomplete combustion.

The coated plate was pressed against a sheet of polymethylmethacrylateunder a pressure of 4000 p. s. i. for a period of 15 minutes at a platentemperature of 350 F.

When the plastic was removed it was found that a coating of carbon wasformed on the surface. The coating was transparent and electricallyconductive. It passed about 65% of incident light, being so transparentthat newsprint could be read through it with ease. It has a resistivityof less than one megohm per square. The film was sufficiently adherentso that Scotch tape could be applied to it and ripped off withoutdamaging the film. The coating could also stand considerable washingwith soap and water and at least a moderate degree of abrasion.

Example 2 A ferrotype plate was coated with carbon black in essentiallythe same way as described in Example 1 except that a considerablyheavier coating was laid down. This coating was next to the top one ofseveral sheets of phenolic impregnated laminating paper. Anotherferrotype plate completed the pile. The whole was pressed at about 2000pounds per square inch and 325 F. for 40 minutes. The carbon wascompletely and firmly transferred to the resulting laminate yielding ahighly abrasion resistant surface with a resistance of 12,000 ohms persquare.

Example 3 The procedure of Example 2 was repeated except that a sheet ofplate glass was substituted for the first mentioned ferrotype plate.Results were substantially the same as reported for Example 2.

The technique described above is not limited to carbon formed by thepyrolysis of carbonaceous fuel gases. Any similarly finely dividedconductive material formed by the pyrolysis of a gasiform substance isequally applicable, as for instance iron or nickel formed by thedecomposition of the corresponding carbonyls or boron formed by thepyrolysis of boron hydrides.

Again, conductive material may be formed by the pyrolysis of liquid orsolid materials which have been spread on the transfer surface. Thuscarbon may be formed by the carbonization of a film of hydrocarbon oilor of. a thin layer of a plastic such as polyethylene orpolydivinylbenzene.

Provided the insulator surface to be coated is sufiiciently resistant toheat, carbon may be deposited directly from a sooty flame of the typedescribed above. Polymonochlorotrifluoroethylcne and glass are suitablefor such treatment. The coated object may then be subjected to heat andpressure sufiicient to secure adherence of the applied coating. Thus, asheet of polymonochlorotrifiuoroethylene may be coated evenly withcarbon black and heated in a press between mirror-finished chrome sheetsto a temperature above its softening point. A glass object after coatingmay be heated under pressure of polished metal sheets in a furnace underneutral or mildly oxidizing conditions to a temperature just above itssoftening point.

Example 4 A sheet of plate glass, 6 inches long and 8 inches wide, wascoated with a thin layer of carbon black by the procedure described inExample 1. The sheet of glass was placed between two /2 thick slabs ofpolished stainless steel and the whole placed on the floor of anelectric furnace. A mass of steel weighing 250 pounds was placed on topof the assembly. The furnace was heated to 1100 F. and held for /2 hour.After cooling, a firmly adherent transparent coating was found on thesurface of the glass. The resistance of the surface was less than onemegohm per square.

A film of the type described herein can be continuously applied to along sheet or strip of plastic by use of the device shown in Figure 3.Roll 34 is of polished chrome plated steel. A sooty flame from burner 36deposits carbon continuously on said roll 34. Plastic sheet 30 passescontinuously between roll 34 and opposing roll 38 being subjected topressure during passage therebetween. An electrical heater or equivalent(not shown) heats the area of roll 34 which is in contact with sheet 30to a temperature somewhat higher than the usual molding temperature ofthe plastic in question. Cooperation of the parts as shown results inthe complete and firm transfer of a carbon film to the surface of theplastic sheet. Continuous sheets of glass may be processed in this manner by preheating the glass to the softening point. Commerciallyavailable apparatus for preheating the glass may be used.

By finely divided material is meant material having a particle size ofmicrons or less and preferably no greater than 20 microns.

While preferred embodiments of the invention have been described, itwill be understood that further modifications may be made withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. The method of applying an electrically conductive transparent coatingof finely divided solid, non-fusible conductive material having aparticle size less than 20 microns to an electrically non-conductivethermoplastic substrate comprising the steps of applying a thintransparent uniform layer of said finely divided material to saidsubstrate followed by the step of applying sufiicient heat and pressureto said finely divided material and said substrate to cause the surfaceof said substrate to soften and said finely divided material to adhereto said softened surface of said substrate.

2. The method of rendering a thermoplastic article electricallyconductive comprising the steps of uniformly coating with carbon havinga particle size less than 20 microns of a surface having low adherenceto carbon terminating the coating step prior to the point at which thecoating becomes opaque, placing said carbon coating into intimatecontact with said plastic article and applying sufiicient pressure andheat to cause said carbon to adhere to said plastic sheet.

3. The method of claim 2 wherein said low adherence surface iselectrostatically charged.

4. The method of claim 2 wherein said plastic article ispolymethylmethacrylate.

5. A process for rendering a thermoplastic article electricallyconductive comprising coating a surface having low adherence for carbonby impinging on said surface the products of combustion from a sootyflame until a continuous but transparent layer of carbon having aparticle size of less than 20 microns is formed, placing said carboncoating into intimate contact with said plastic article and applyingsufiicient heat and pressure to cause said carbon to adhere to saidplastic article;

6. The process of claim 5 wherein said sooty flame is produced by theburning of acetylene gas.

7. The method of claim 1 wherein said substrate is 2,221,77 glass.2,328,422 8. The method of claim I wherein said material is 2,357,809applied to said substrate by first applying said material ,4 3, 13 to asurface having low adhesion for said material, and 5 0 placing saidcoated surface into intimate contact with said substrate.

References Cited in the file of this patent UNITED STATES PATENTS 101,987,969 Parkin Ian. 15, 1935 6 Carlson Nov. 19, 1940 Christensen Aug.31, 1943 Carlson Sept. 12, 1944 Von Hippel a. D60. 24, 1946 Simpson May30. 1950 OTHER REFERENCES Printed Circuit Techniques, Nat. Bur. Stand.Circ. 468, November 15, 1947.

1. THE METHOD OF APPLYING AN ELECTRICALLY CONDUCTIVE TRANSPARENT COATINGOF FINELY DIVIDED SOLID, NON-FUSIBLE CONDUCTIVE MATERIAL HAVING APARTICLE SIZE LESS THAN 20 MICRONS TO AN ELECTRICALLY NON-CONDUCTIVETHERMOPLASTIC SUBSTRATE COMPRISING THE STEPS, OF APPLYING A THINTRANSPARENT UNIFORM LAYER OF SAID FINELY DIVIDED MATERIAL TO SAIDSUBSTRATE FOLLOWED BY THE STEP OF APPLYING SUFFICIENT HEAT AND PRESSURETO SAID FINELY DIVIDED MATERIAL AND SAID SUBSTRATE TO CAUSE THE SURFACEOF SAID SUBSTRATE TO SOFTEN AND SAID FINELY DIVIDED MATERIAL TO ADHERETO SAID SOFTENED SURFACE OF SAID SUBSTRATE.